The Mossack Fonseca files feature heads of government, politicians and their relatives, as well as MPs, peers and thousands of wealthy individuals.
Photograph: Wolfgang Rattay/Reuters

UK tax investigators have written to media organisations, including the Guardian, to request access to the huge trove of 11.5m secret offshore files leaked from the Panamanian law firm Mossack Fonseca.

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The files feature heads of government, monarchs, politicians, and their relatives, as well as MPs, peers and thousands of wealthy individuals. Some live in the UK; others make use of companies registered in British tax havens such as Jersey, Guernsey, and the Isle of Man, or in overseas territories such as the British Virgin Islands.

The Australian Tax Office (ATO) said it was investigating more than 800 wealthy clients of Mossack Fonseca. “We have now linked over 120 of them to an associate offshore service provider located in Hong Kong,” the Australian tax office said in a statement. It did not name the Hong Kong company.

The 800 individuals under investigation include taxpayers who had previously been investigated. It also included others who had reported themselves to the tax office under a voluntary disclosure initiative that allowed people to come forward and avoid steep penalties and criminal charges. The scheme has since ended.

However, the ATO said those under investigation also included many taxpayers who had not previously come forward.

In the UK, Vicky Ranson of Her Majesty’s Revenue and Customs (HMRC), said tax officials were keen to obtain the data. In a letter to the Guardian’s editor-in-chief, Katharine Viner, she said HMRC was “relentlessly pursuing tax evasion by UK citizens and residents” and wanted“to ensure that tax cheats have no safe havens in which to hide undeclared income”.

Ranson, director of HMRC’s risk and intelligence service, added: “I am very keen to obtain this data from you so we can cross-reference it with our own extensive data, to see whether there is information that we do not have and which could be useful as part of our extensive investigations into offshore tax evasion.”

In a statement, the Guardian said: “We have received the request from HMRC and will consult with our international partners.”

The UK’s shadow chancellor, John McDonnell, on Monday urged the government to do more to tackle offshore tax evasion. “Cameron promised and has failed to end tax secrecy and crack down on ‘morally unacceptable’ offshore schemes. Real action is now needed.”

New Zealand’s tax agency said it was working closely with its tax treaty partners to obtain full details of any New Zealand tax residents who may have been involved in arrangements facilitated by Mossack Fonseca.

The 11.5m documents come from Mossack Fonseca’s internal database and include company registration forms, emails, passport copies, and correspondence with banks. They reveal the secret beneficial owners – the real owner of a company, even though their name may not appear on the shareholder register– of some offshore companies.

The Panamanian firm previously suffered a smaller leak from its branch office in Luxembourg. German tax authorities bought the data for around €1m (£800,000) and in February 2015 carried out a series of raids across the country on more than 100 people suspected of dodging taxes.

Several European countries, including the UK, obtained the same Luxembourg dataset, it is understood. It is unclear what action,if any, was taken by HMRC.

The latest Mossack Fonseca leak is much larger – a potential goldmine for HMRC inspectors that covers 214,448 companies.

In previous cases HMRC has attracted criticism for failing to prosecute wealthy individuals caught evading tax. Hervé Falciani, a former IT worker, leaked details of 130,000 holders of secret accounts with HSBC’s Swiss private bank. He was convicted in his absence for the biggest leak in banking history, and is currently living in France, having fled Switzerland.